The invention relates to a system for the power control of high voltage electron beam generators having a cathode and a control electrode associated with the cathode, said system consisting of a high voltage isolation transformer with primary and secondary winding to supply the control electrode with a control voltage, and having a device connected to the primary side of the isolation transformer for the production of square-wave pulses susceptible of amplitude modulation.
Electron beam generators of the above described kind are used in many fields of technology, but principally for the melting, welding, machining and vaporization of materials of all kinds, preferably under a vacuum. For the purpose of varying the intensity of an electron beam, the heating power of the incandescent cathode, the bias voltage of the control electrode, and/or the applied acceleration voltage can be varied.
In high voltage electron beam generators, which are generally operated at an anode voltage above 20 kilovolts and which therefore have a relatively low beam current at a given output power, there is provided in the vicinity of the cathode a cylindrical control electrode which is often referred to also as a Wehnelt cylinder. With this electrode it is possible to vary the beam current between 0 and 100%. For this purpose, a voltage of variable amplitude which is negative in relation to the cathode is applied to the control electrode and suppresses the emergence of the electrons in the direction of the anode to a greater or lesser degree according to the magnitude of this control voltage. The voltage level for the secure blocking of the electron beam generator is several kilovolts depending on the construction of the control electrode. The maximum beam current is achieved when the cathode and control electrode are at the same potential.
German "Auslegeschrift" 1,131,760 has previously disclosed a system of the kind described above, in which the control electrode is periodically fed with impulses which are superimposed upon a direct current bias voltage. This is intended to cause the electron beam to be turned on intermittently. What is involved, therefore, is a constant turning on and off of an electron beam of always constant power, the power level being, of course, variable. The power can also be controlled in this case by the relative time of operation of the electron beam. In the known system, however, no direct current component of the control voltage is transferred to the control electrode on account of the isolation transformer that is present, so that no continuous control of the power of the electron beam generator is possible.
Continuous power control, however, is necessary for a number of purposes, such as, for example, for increasing the power in electron beam welding at the beginning and at the end of a seam, in the welding of workpieces of different material thicknesses, etc. The impulse frequency, furthermore, cannot be lower than about 50 to 100 Hz on account of the band width of the isolation transformer. But, in particular, the known system does not permit a distortion-free transfer of the impulses through the isolation transformer at the lower end of the band of frequencies to be transmitted. Instead, on the secondary side, a "tilt" will occur, which will increase as the length of the pulses increases, even though perfect square wave pulses are put into the primary side of the isolation transformer. The reason for this is the dynamic behavior of the magnetic coupling of the two circuits within the transformer. The source of the difficulty is thus a basically unavoidable property of magnetically coupled circuits.